Image processing apparatus for carrying out tone conversion processing and color correction processing using a three-dimensional look-up table

ABSTRACT

When tone conversion processing and color correction processing is carried out on image data obtained by a digital camera, processed image data enabling reproduction of a high quality image can be obtained. A 3DLUT for carrying out the tone conversion processing and the color correction processing on the image data obtained by the digital camera is generated. The image data are converted according to the 3DLUT, and subjected to sharpness processing to generate the processed image data. The processed image data are printed as a print by a printer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image processing method and an imageprocessing apparatus for carrying out tone conversion processing andcolor correction processing on digital image data obtained by a digitalcamera. The present invention also relates to a computer-readablerecording medium storing a program to cause a computer to execute theimage processing method.

2. Description of the Related Art

In a digital still camera (hereinafter called digital camera), an imageobtained by photographing is recorded as digital image data in aninternal memory installed in the camera or in a recording medium such asan IC card, and the image can be displayed by using a monitor or aprinter based on the recorded digital image data. When an image obtainedby a digital camera is printed, the image is expected to have as high aquality as a photograph generated from a negative film.

Meanwhile, in the field of printing, a system for outputting an image asa hard copy from a printer by reading a color image with a scanner toobtain input image data and by carrying out a desired kind of imageprocessing on the input image data to generate output image data hasbeen put into practice (Japanese Unexamined Patent Publication No.11(1999)-234523, for example). In such a system, the input image dataare converted from RGB color signals into CMYK dot % signals. First, atone curve (a tone conversion table) and a color correction amount usedin a color correction unit and the like are set for the input imagedata, and a three-dimensional look-up table (hereinafter called 3DLUT)for converting the input image data into the output image data isgenerated based on the tone curve and the color correction amount thathave been set. The RGB color signals of the input image data areconverted into the CMYK dot % signals of the output image data byinterpolating the 3DLUT. Printing is carried out by controlling anamount of ink of each color by the dot % signal.

Meanwhile, tone conversion and color correction is also carried out whenimage data obtained by a digital camera are printed. As in the systemfor printing, conditions for the tone conversion and the colorcorrection are also set for each image, and reproduction of a highquality image is also expected.

SUMMARY OF THE INVENTION

The present invention has been conceived based on consideration of theabove problem. An object of the present invention is therefore toprovide an image processing method and an image processing apparatus forobtaining processed image data having a higher quality by carrying outtone conversion processing and color correction processing on image dataobtained by a digital camera, and to provide a computer-readablerecording medium storing a program to cause a computer to execute theimage processing method.

A first image processing method of the present invention is a method forobtaining processed image data by carrying out tone conversionprocessing and color correction processing on image data obtained by adigital camera, and the image processing method comprises the steps of:

generating a three-dimensional look-up table for carrying out the toneconversion processing and the color correction processing on the imagedata; and

obtaining the processed image data by converting the image dataaccording to the three-dimensional look-up table.

In the first image processing method of the present invention, it ispreferable for the three-dimensional look-up table to be generated for amodel of the digital camera.

In a digital camera, tone processing, automatic exposure controlprocessing (AE processing) and automatic white balance processing (AWBprocessing) is carried out on image data obtained by the camera.Therefore, generating “the three-dimensional look-up table for a modelof the digital camera” refers to generating the three-dimensionallook-up table enabling elimination of an effect of tone processingcarried out in the digital camera.

A three-dimensional look-up table has a predetermined number of latticepoints. However, it is preferable for the number of lattice points inthe three-dimensional look-up table of the present invention to be setin accordance with the number of bits of the image data.

If the number of lattice points of the three-dimensional look-up tableis the same as the number of bits of the image data (for example, 256³in the case where the image data comprise RGB data having 8 bits each),operation time for generating the look-up table becomes long. On theother hand, if the number of lattice points is too small, accuracy ofoperation for interpolating the lattice points may be lowered at thetime of converting the image data. Therefore, setting “the number oflattice points in accordance with the number of bits” refers to settingthe operation time for calculating the three-dimensional look-up tableand the accuracy of the operation for interpolating the lattice points.For example, in order to improve the accuracy according to the number ofbits of the image data, the number of the lattice points is set to(2^(n)/8+1) where n is the number of bits of the image data.

Furthermore, in the first image processing method of the presentinvention, it is preferable for the number of pixels in an imagerepresented by the image data to be compared with the number of thelattice points in the three-dimensional look-up table. In this case, theprocessed image data are obtained by generating the three-dimensionallook-up table and by converting the image data according to the look-uptable in the case where the number of the pixels is larger than thenumber of the lattice points; and

by carrying out the tone conversion processing and the color correctionprocessing on each of the pixels in the image represented by the imagedata in the case where the number of the pixels is equal to or smallerthan the number of the lattice points.

“Carrying out the tone conversion processing and the color correctionprocessing on each of the pixels” refers to carrying out the toneconversion processing and the color correction processing on each of thepixels according to a predetermined operational expression and not usingthe three-dimensional look-up table.

It is preferable for the image data to have a file format having taginformation, such as Exif or JPEG. In this case, it is preferable forthe number of bits of the image data and the number of the pixels to bedescribed in the tag information.

A second image processing method of the present invention is a methodfor obtaining processed image data by carrying out tone conversionprocessing and color correction processing on image data, and the methodcomprises the steps of:

comparing the number of lattice points in a three-dimensional look-uptable used for carrying out the tone conversion processing and the colorcorrection processing on the image data with the number of pixels in animage represented by the image data;

generating the three-dimensional look-up table and obtaining theprocessed image data by converting the image data according to thethree-dimensional look-up table in the case where the number of thepixels is larger than the number of the lattice points; and

obtaining the processed image data by carrying out the tone conversionprocessing and the color correction processing on each of the pixels inthe image represented by the image data, in the case where the number ofthe pixels is equal to or smaller than the number of the lattice points.

In the second image processing method of the present invention, it ispreferable for the number of the lattice points in the three-dimensionallook-up table to be set according to the number of bits of the imagedata.

A first image processing apparatus of the present invention is anapparatus for obtaining processed image data by carrying out toneconversion processing and color correction processing on image dataobtained by a digital camera, and the apparatus comprises:

three-dimensional look-up table generating means for generating athree-dimensional look-up table used for carrying out the toneconversion processing and the color correction processing on the imagedata; and

processing means for obtaining the processed image data by convertingthe image data according to the three-dimensional look-up table.

It is preferable for the three-dimensional look-up table generatingmeans of the first image processing apparatus of the present inventionto generate the three-dimensional look-up table according to a model ofthe digital camera.

It is also preferable for the three-dimensional look-up table generatingmeans to set the number of lattice points of the three-dimensionallook-up table according to the number of bits of the image data.

Furthermore, it is preferable for the three-dimensional look-up tablegenerating means to compare the number of pixels of an image representedby the image data with the number of lattice points in thethree-dimensional look-up table. If the number of the pixels is largerthan the number of the lattice points, the three-dimensional look-uptable generating means generates the three-dimensional look-up table,and the processing means obtains the processed image data by convertingthe image data according to the three-dimensional look-up table. On theother hand, if the number of the pixels is equal to or smaller than thenumber of the lattice points, the processing means obtains the processedimage data by carrying out the tone conversion processing and the colorcorrection processing on each of the pixels of the image represented bythe image data.

A second image processing apparatus of the present invention is anapparatus for obtaining processed image data by carrying out toneconversion processing and color correction processing on image data, andthe second image processing apparatus comprises:

three-dimensional look-up table generating means for comparing thenumber of lattice points in a three-dimensional look-up table used forthe tone conversion processing and the color correction processing onthe image data with the number of pixels in an image represented by theimage data, and for generating the three-dimensional look-up table inthe case where the number of the pixels is larger than the number of thelattice points; and

processing means for obtaining the processed image data by convertingthe image data according to the three-dimensional look-up table in thecase where the number of the pixels is larger than the number of thelattice points, and for obtaining the processed image data by carryingout the tone conversion processing and the color correction processingon each of the pixels in the image represented by the image data, in thecase where the number of the pixels is equal to or smaller than thenumber of the lattice points.

In the second image processing apparatus of the present invention, it ispreferable for the three-dimensional look-up table generating means toset the number of the lattice points in the three-dimensional look-uptable according to the number of bits of the image data.

The image processing methods of the present invention may be provided asa computer-readable recording medium storing programs to cause acomputer to execute the image processing methods.

According to the present invention, the three-dimensional look-up tableused for carrying out tone conversion processing and color correctionprocessing on the image data obtained by the digital camera isgenerated, and the processed image data are obtained by converting theimage data according to the table. Therefore, appropriate toneconversion processing and color correction processing can be carriedout, and the processed image data enabling reproduction of ahigh-quality image can be obtained.

Furthermore, by generating the three-dimensional look-up table for themodel of the digital camera, the processed image data enablingreproduction of a high-quality image without an effect of toneprocessing in the digital camera can be obtained, regardless of themodel of the digital camera.

Moreover, by determining the number of the lattice points in thethree-dimensional look-up table according to the number of bits of theimage data, operation time for generating the three-dimensional look-uptable and accuracy of operation for interpolating the lattice points atthe time of image data conversion can be controlled. In this manner, theimage data can be converted quickly or accurately.

In the case where the number of the pixels in the image represented bythe image data is larger than the number of the lattice points in thethree-dimensional look-up table, the three-dimensional look-up table isgenerated, and the processed image data are obtained by converting theimage data according to the three-dimensional look-up table. On theother hand, if the number of the pixels is equal to or smaller than thenumber of the lattice points, the processed image data are obtained bycarrying out the tone conversion processing and the color correctionprocessing on each of the pixels. In this manner, the tone conversionprocessing and the color correction processing can be carried out on theimage data by a smaller amount of calculation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a configuration of an image outputapparatus comprising an image processing apparatus as an embodiment ofthe present invention;

FIG. 2 is a block diagram showing a configuration of image processingcondition determining means;

FIG. 3 is a diagram explaining setting of a tone conversion table;

FIG. 4 shows tables of color correction condition setting menus;

FIG. 5 is a diagram showing an example of an intensity function;

FIG. 6 is a diagram showing an example of an intensity function for askin color;

FIG. 7 shows an index image displayed on a monitor;

FIG. 8 shows an additional color correction condition setting menu; and

FIG. 9 is a flow chart showing operation of this embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Hereinafter, an embodiment of the present invention will be explainedwith reference to the accompanying drawings.

FIG. 1 is a block diagram showing a configuration of an image outputapparatus adopting an image processing apparatus as the embodiment ofthe present invention. As shown in FIG. 1, an image output apparatus 1in this embodiment comprises reading means 3 for reading, from a memorycard 2, image data S0 comprising color data R0, G0, and B0 obtained byphotographing a subject with a digital camera, index image generatingmeans 4 for generating index image data S11 representing index images byreducing the image data S0, setting information generating means 5 forgenerating setting information H0 necessary for setting tone conversiontables T0 which will be explained later by analyzing the image data S0,image processing condition determining means 6 for setting imageprocessing conditions used for carrying out tone conversion processingand color correction processing on the image data S0 upon printing theimage data S0 and for carrying out tone conversion processing on theindex image data S11, a monitor 7 for displaying index image data S11′after the tone conversion processing as index images, input means 8 forcarrying out various kinds of inputs to the image processing conditiondetermining means 6, DCMY keys 9 for changing density, processing means10 for obtaining converted image data S12 by converting the image dataS0 according to a 3DLUT or the image processing conditions generated orset by the image processing condition determining means 6, reductionmeans 11 for obtaining reduced image data S0′ by reducing the image dataS0 in the case where the number of pixels of an image represented by theimage data S0 is larger than the number of pixels necessary for printingthe image, enlargement means 12 for obtaining enlarged image data S12′by enlarging the converted image data S12 in the case where the numberof the pixels of the image data S0 is equal to or smaller than thenumber of the pixels necessary for printing, sharpness processing means13 for obtaining processed image data S13 by carrying out sharpnessprocessing on the converted image data S12 or on the enlarged image dataS12′, and a printer 14 for obtaining a print P by outputting theprocessed image data S13.

In the following explanation, processing is carried out on the reducedimage data S0′ in the case where the image data S0 are reduced by thereduction means 11. However, for the sake of simplicity, the processingis carried out only on the image data S0 in the explanation below, andexplanation regarding the processing on the reduced image data S0′ isomitted.

The reading means 3 comprises a card reader and the like for reading theimage data S0 from the memory card 2. Since the image data read from thememory card 2 are generally compressed, the reading means 3 comprisesdecompression means not shown, and the image data S0 are obtained bydecompressing the image data read from the memory card 2 by using thedecompression means. Information indicating a model of the digitalcamera that obtained the image data S0 (hereinafter called camera modelinformation) is added to the image data S0 as tag information thereof.Therefore, the camera model information is also read. As a standard forrecording the camera model information as the tag information, “BaselineTIFF Rev.6.0 RGB Full Color Image” adopted as a non-compressed fileformat of an Exif file can be used, for example. In the case where theimage data S0 do not have the camera model information, the camera modelinformation can be input manually from the input means 8.

The index image generating means 4 reduces the image data S0 by thinningor the like, and generates the index image data S11.

The setting information generating means 5 generates the settinginformation H0 in the following manner. In general, automatic exposurecontrol processing (AE processing) and automatic white balanceprocessing (AWB processing is carried out in a digital camera forreproduction of image data on a monitor. However, in the case where theimage data are reproduced by a printer, the AE processing and the AWBprocessing (hereinafter called AE/AWB processing) carried out in thedigital camera is not sufficient. Therefore, AE/AWB processingappropriate for printing is necessary. The setting informationgenerating means 5 estimates a correction amount necessary for AE/AWBprocessing optimal for printing for each of RGB color signals comprisingthe image data S0. The correction amount is included in the settinginformation H0. For this reason, an average of each of the RGB colorsignals comprising the image data S0 is found as described in JapaneseUnexamined Patent Publication No. 11(1999)-220619, and a correctionvalue is determined so that the average becomes a target valueappropriate for printing. The correction value is included as thecorrection amount in the setting information H0 and output. Thecorrection amount is for correcting exposure and white balance.

The setting information generating means 5 finds a correction amountused for correcting highlight and shadow of tones into a nonlinear formwhen the image processing condition determining means 6 determines theimage processing conditions that will be explained later. Thiscorrection amount is also included in the setting information H0. Aprinter generally has a narrow density reproduction range, and colorseasily become evenly bright in a highlight portion and evenly dark in ashadow portion of an image. Therefore, as described in JapaneseUnexamined Patent Publication No. 11(1999)-331596, the settinginformation generating means 5 finds the correction amount to hardentones of the highlight portion and soften tones of the shadow portion,in the case where density of the print increases by the AE processing orthe AWB processing. On the contrary, in the case where the densitydecreases in the print, the setting information generating means 5 findsthe correction amount so that the tones of the highlight portion aresoftened and the tones of the shadow portion are hardened. Thecorrection amount is also included in the setting information H0.

The setting information generating means 5 reads the tag information ofthe image data S0, and includes the camera model information of the taginformation and the number of pixels (hereinafter called Y0) of theimage represented by the image data S0 in the setting information H0. Inthe case where flash information is included in the tag information, theflash information is also included in the setting information H0.

The monitor 7 displays the index images represented by the index imagedata S11′. At the time of correcting tone curves and color correctionconditions that will be explained later, the tone curves and the colorcorrection conditions are also displayed on the monitor together withthe index images. In this embodiment, the number of the index imagesdisplayed simultaneously is six.

The input means 8 comprises a keyboard and a mouse for carrying outvarious kinds of inputs to the image processing condition determiningmeans 6. A type of tones used as a reference at the time of determiningthe image processing conditions (hereinafter called reference tones) isinput from the input means 8. The reference tones refer to tones forcarrying out tone conversion processing on the image data to cause theprint P to have appropriate tones when printing is carried out by theprinter 14. As the reference tones, tones for a standard situation,tones for cloudy weather, tones for backlight, and tones for flashphotographing are selectable. By selection of one of the types of thereference tones input from the input means 8, the image processingcondition determining means 6 sets a reference tone curve representingthe selected reference tones. If correction of the tone curves includingthe reference tone curve is needed to obtain desired tones, the tonecurves are shown on the monitor 7 and corrected by an input from theinput means 8.

The DCMY keys 9 comprise four keys for correcting density D of theentire image and for correcting densities of cyan (C) magenta (M) andyellow (Y), respectively. Depending on how many times each of these keysis pressed down, the density of the entire image or each of the colorscan be changed by the image processing condition determining means 6.Correction of the reference tone curve representing the reference tonesinput from the input means 8 and the change of the densities input fromthe DCMY keys 9 are reflected in the index images displayed on themonitor 7 in real time.

The image processing condition determining means 6 sets the imageprocessing conditions for carrying out the tone conversion processingand the color correction processing on the image data S0. Depending onthe number Y0 of the pixels in the image represented by the image dataS0 included in the tag information of the image data S0, the imageprocessing condition determining means 6 outputs to the processing means10 either the image processing conditions as they are, or the 3DLUTgenerated therein based on the image processing conditions. In the imageoutput apparatus in this embodiment, the image processing conditiondetermining means 6 compares the number Y0 with the number of latticepoints in the 3DLUT, in order to shorten processing time. In the casewhere the number Y0 of the pixels is larger than the number of thelattice points, the 3DLUT is generated and output as the imageprocessing conditions to the processing means 10. On the other hand, ifthe number Y0 of the pixels is equal to or smaller than the number ofthe lattice points, the image processing conditions are output to theprocessing means 10 as they are, and the 3DLUT is not generated.Hereinafter, the image processing condition determining means 6 will beexplained in detail, with reference to FIG. 2.

FIG. 2 is a block diagram showing a configuration of the imageprocessing condition determining means 6. In the case where the imagedata S0 comprise 8-bit RGB color data upon generating the 3DLUT, 256³data will be necessary for the 3DLUT if all the data are to beconverted. Consequently, generation of the 3DLUT becomes time-consuming.Therefore, in this embodiment, the 3DLUT is generated by reducing thenumber of bits of each of the color data R0, G0, and B0 so that thenumber of the lattice points becomes (2^(n)/8+1) where n is the numberof bits of the image data S0. For example, in the case where the imagedata S0 comprise the RGB color data having 8 bits each, the number ofbits of each of the color data is reduced to generate the 3DLUTcomprising 33³ lattice points having 33 data for each color, namely 0,7, 15, . . . , 247 and 255.

As shown in FIG. 2, the image processing condition determining means 6comprises logarithmic conversion means 21 for obtaining image data S1 bycarrying out logarithmic conversion on antilogarithmic color data R0′,G0′, and B0′ representing colorimetric values of the subject itself andfound from the image data S0 (having the reduced number of bits)according to ITU-R BT.709 (REC. 709) by using Equations (1)˜(3) below,tone conversion means 22 for obtaining image data S2 by carrying out thetone conversion processing on the image data S1 after the logarithmicconversion, tone setting means 23 for setting the tone conversion tablesT0 used for the tone conversion by the tone conversion means 22, amemory 24 storing a plurality of tone curves, inverse logarithmicconversion means 25 for obtaining image data S3 comprising color dataR3, G3, and B3 by carrying out inverse logarithmic conversion on theimage data S2, LCH conversion means 26 for converting the color data R3,G3, and B3 comprising the image data S3 into data L3, C3 and H3representing lightness L*, chroma C*, and hue angle HA, color correctionmeans 27 for obtaining color corrected data L4, C4, and H4 by carryingout the color correction processing on the data L3, C3, and H3, sRGBconversion means 28 for obtaining color corrected image data S4comprising color data R4, G4, and B4 by converting the color correcteddata L4, C4 and H4 into an sRGB color space which is a color space formonitor display, printing conversion means 29 for obtaining print imagedata S5 by converting the color corrected image data S4 into a colorspace for printing, LUT generating means 30 for generating the 3DLUTbased on the image data S0 and the print image data S5, and processingmethod determining means 33 for determining whether the image processingconditions or the 3DLUT are output to the processing means 10. The colorcorrection means 27 is connected to color correction condition settingmeans 32 for setting the color correction conditions used by the colorcorrection means 27. The color correction condition setting means 32reads the color correction conditions corresponding to both the settinginformation H0 and the input from the input means 8 from a memory 31storing a plurality of reference color correction condition settingmenus representing color correction conditions used as references andmodel color correction condition setting menus representing colorcorrection conditions corresponding to models of digital camera. Thecolor correction condition setting means 32 customizes the colorcorrection conditions if necessary, and inputs the conditions to thecolor correction means 27.Pr=R0/255Pg=G0/255Pb=B0/255  (1)R0′=((Pr+0.099)/1.099)^(2.222)G0′=((Pg+0.099)/1.099)^(2.222) (if Pr, Pg, Pb≧0.081)B0′=((Pb+0.099)/1.099)^(2.222)  (2)R0′=Pr/4.5G0′=Pg/4.5 (if Pr, Pg, Pb<0.081)B0′=Pb/4.5  (3)

The memory 24 stores reference tone curves comprising tone curves forthe standard situation, cloudy weather, backlight, and flashphotographing, as well as tone curves corresponding to models of digitalcamera.

The tone conversion tables T0 are set by the tone setting means 23, inorder to convert tones of the image data S1 in the following manner.FIG. 3 is a diagram for explaining how the tone conversion tables T0 areset. The tone conversion tables T0 generate the color data R2, G2, andB2 comprising the image data S2 by the tone conversion of the color dataR1, G1, and B1 comprising the image data S1 according to first to fourthquadrants in FIG. 3. In the tone setting means 23, the tone conversiontables T0 are set for each of the RGB colors. First, the settinginformation H0 is input to the tone setting means 23, and one of thetone curves corresponding to the model of the digital camera is readfrom the memory 24 based on the information on the model of the digitalcamera included in the setting information H0. Meanwhile, the referencetone curve for the standard situation is read from the memory 24 as adefault of the reference tone curves. However, if reading the referencetone curve for cloudy weather or for backlight or for flashphotographing is instructed by the input means 8, the corresponding tonecurve is read from the memory 24.

A tone curve C1 corresponding to the model of the digital camera is setin the first quadrant as shown in FIG. 3. Since a quality of areproduced image varies depending on a model or a manufacturer of adigital camera, the tone curve C1 has been generated for the model ofthe digital camera to absorb a tone characteristic of the camera. Byusing the tone curve C1, images of uniform quality can be obtainedregardless of the model of the camera. By converting the color data R1,G1 and B1 based on the tone curve C1, data representing a logarithmicexposure value compensating the tone characteristic of the camera can beobtained.

A line C2 for correcting exposure is set in the second quadrant. Theline C2 for exposure correction is basically a line passing through theorigin. By translating the line C2 in directions shown by an arrow Abased on the correction amount for exposure and white balance includedin the setting information H0, the exposure is corrected. The AE/AWBprocessing appropriate for printing is carried out based on the line C2,and data representing reflection density of the subject are obtained.

A reference tone curve C3 is set in the third quadrant. In this example,the reference tone curve C3 for the standard situation is set. Thereference tone curve C3 has an “S”-like shape and a middle portionthereof corresponds to the case of γ=1.6. In this embodiment, theconversion by the reference tone curve C3 is called γ conversion. Byusing the tone curve C3, density data appropriate for printing can beobtained.

A tone curve C4 for correcting the tones of the highlight portion andthe shadow portion of the image to become nonlinear is set in the fourthquadrant. A correction amount for the tone curve C4 is determined by thecorrection amount of the highlight portion and the shadow portionincluded in the setting information H0. By using the tone curve C4, thecolor data R2, G2 and B2 comprising the image data S2 can be obtained.

The tone conversion tables T0 are changed by an input from the inputmeans 8 or from the DCMY keys 9. C, M, and Y of the index images shownon the monitor 7 can be changed by pressing down the DCMY keys 9.Changes in C, M, and Y are converted into changes of densities of R, G,and B, and the tone conversion tables T0 are changed according to thechanges in the densities. In other words, the changes of RGB densitiesare predetermined depending on how many times the DCMY keys 9 arepressed, and the densities are changed by pressing the DCMY keys 9. Morespecifically, by translating the line C2 in the directions shown by thearrow A in the second quadrant based on how many times the DCMY keys arepressed, the densities of R, G, and B are changed. Furthermore, the tonecurve Cl in the first quadrant or γ of the tone curve C3 in the thirdquadrant is changed by the input from the input means 8. In this case,the tone curves C1 and C3 for each of the colors are displayed togetherwith the index images on the monitor 7, and a user changes the tonecurves C1 and C3 as desired with the input means 8 while viewing theindex images. By changing the tone curve C1, the line C2 and/or the tonecurve C3, the tone conversion tables T0 are set.

The tone conversion means 22 obtains the image data S2 by converting theimage data S1 based on the tone conversion tables T0 set by the tonesetting means 23.

Processing in the logarithmic conversion means 21, the tone conversionmeans 22, and the inverse logarithmic conversion means 25 is carried outin an RGB color space.

The LCH conversion means 26 converts the image data S3 from the RGBcolor space into an L*a*b* color space, and obtains the data L3, C3, andH3 representing the lightness L*, the chroma C*, and the hue angle HA.Hereinafter, this conversion will be explained. Since the image data S0obtained by the digital camera are according to ITU-R BT.709 (REC 709),the color data R3, G3, and B3 comprising the image data S3 are convertedinto CIE1931 tristimulus values X, Y, and Z based on the followingEquation (4):X R3Y=|A|G3Z B3  (4)Here, the matrix |A| is a matrix for converting the color data R3, G3,and B3 into the tristimulus values X, Y, and Z, and the following valuescan be used, for example:0.4124 0.3576 0.1805|A|=0.2126 0.7152 0.07220.0193 0.1192 1.0571  (5)Instead of the matrix |A|, a look-up table may be used to find thetristimulus values X, Y, and Z.

CIE1976 L*(=L3), C*(=C3), and HA (=H3) are found from the tristimulusvalues X, Y, and Z, based on Equations (6)˜(8) below:a*=500{f(X/Xn)−f(Y/Yn)}b*=200{f(Y/Yn)−f(Z/Zn)}L*=116(Y/Yn)^(1/3)−16 (if Y/Yn>0.008856)L*=903.25(Y/Yn)(if Y/Yn≦0.008856)  (6)If X/Xn, Y/Yn, Z/Zn>0.008856,f(a/an)=(a/an)^(1/3)(a=X, Y, Z)If X/Xn, Y/Yn, Z/Zn≦0.008856,f(a/an)=7.787(a/an)+16/116Xn, Yn, and Zn are tristimulus values for white and can be substitutedby tristimulus values corresponding to CIE-D65 (a light source whosecolor temperature is 6500 K.).C*=(a* ² +b* ²)^(1/2)  (7)HA=tan⁻¹(b*/a*)  (8)

The color correction means 27 corrects lightness, chroma and hue ofeleven colors, namely R, G, B, C, M, Y, YellowGreen (YG), BlueSky (BS),a skin color SK (HL) in the highlight portion, a skin color SK(MD)having an intermediate density, and a skin color SK(SD) in the shadowportion. More specifically, the corrected data L4, C4, and H4 areobtained by correcting the data L3, C3 and H3 according to Equations(9)˜(11) below:L4=L3−ΔLΔL=ΣLPi Wi+ΣLPj Wj+Δl Wj  (9)C4=C3−ΔCΔC=ΣCPi Wi+ΣCPj Wj+Δc Wj  (10)H4=H3−ΔHΔH=ΣHPi Wi+ΣHPj Wj+Δh WJ  (11)where ΔL: a correction value for the lightness,

ΔC: a correction value for the chroma,

ΔH: a correction value for the hue,

i: R, G, B, C, M, Y, YG, BS,

j: SK(HL), SK(MD), SK(SD),

LPi, LPj: correction degrees in the lightness,

CPi, CPj: correction degrees in the chroma,

HPi, HPj: correction degrees in the hue,

Wi, Wj: intensity functions,

Δl: a lightness change caused by tone conversion,

Δc: a chroma change caused by tone conversion, and

Δh: a hue change caused by tone conversion.

The lightness correction degrees LPi and LPj, the chroma correctiondegrees CPi and CPj, and the hue correction degrees HPi and HPj areprovided by the color correction condition setting means 32. The colorcorrection condition setting means 32 selects a desired one of referencecolor correction conditions in the reference color correction conditionsetting menu and one of model color correction conditions in the modelcolor correction condition setting menu corresponding to the model ofthe digital camera read from the reference color correction conditionsetting menus and the model color correction condition setting menusstored in the memory 31. The color correction condition setting means 32customizes the color correction conditions if necessary, and inputs theconditions to the color correction means 27. When the settinginformation H0 is input to the color correction condition setting means32, the model color correction condition setting menu corresponding tothe model of the digital camera is read from the memory 31 based on theinformation on the model of the digital camera included in the settinginformation H0. Meanwhile, a reference color correction conditionsetting menu for the standard situation is read from the memory 31 as adefault menu. In the case where reading a color correction conditionsetting menu for cloudy weather or for backlight or for flashphotographing is specified by the input means 8, a color correctioncondition setting menu therefor is read from the memory 31.

The reference color correction condition setting menu and the modelcolor correction condition setting menu can be customized by changingthe selected conditions as desired with the input means 8. The referencecolor correction condition setting menu and the model color correctioncondition setting menu may be stored in the memory 31 by beingclassified according to users, such as “a cloudy-weather reference colorcorrection condition setting menu for user A” and “a cloudy-weathermodel color correction condition setting menu for digital camera A ofuser A”.

The color correction condition setting menus have a plurality of valuesrepresenting how much the lightness, the chroma and the hue need to bechanged. The color correction means 27 sets the lightness correctiondegrees LPi and LPj, the chroma correction degrees CPi and CPj, and thehue correction degrees HPi and HPj in Equations (9)˜(11) according tothe values set in the reference color correction condition setting menuand the model color correction condition setting menu. The correctiondegrees in each color are obtained by adding the corresponding values inthe reference color correction condition setting menu and the modelcolor correction condition setting menu.

The intensity functions are found according to Equation (12) below:Wi=F(d)where d=((Li−L)²+(ai−a)²+(bi−b)²))^(1/2)  (12)Li, ai, and bi refer to center colors of R, G, B, C, M, Y, YG, and BS inthe L*a*b* color space. For R, G, B, C, M, and Y, the center colorsrefer to calorimetric values of the colors in the Macbeth Color Checkerregistered by Macbeth A Division of Kallmorgen Co. For YG and BS, thecenter colors refer to average calorimetric values of vegetation and skyin the image represented by the image data S0. d is a distance in theL*a*b* color space between the center color Li, ai, and bi of each ofthe colors and the values of L*, a*, and b* obtained by the LCHconversion means 26. F(d) is a function such that F(d) is constant up toa predetermined value of d (30 in this example) and decreases if dbecomes larger than the predetermined value, as shown in FIG. 5.

Meanwhile, the intensity function Wj is a function for skin color.Statistical distribution ranges of the skin colors SK(HL), SK(MD) andSK(SD) of the image represented by the image data S0 in the L*a*b* colorspace are found, and the skin color function W(j) [0≦Wj≦1] is set insuch a manner that the value of the function is small in a peripheralarea and large at a center area in the distributions, as shown in FIG.6.

As shown in FIG. 7, in any one of the index images shown on the monitor7, any color other than R, G, B, C, M, Y, YG, BS, SK(HL), SK(MD), andSK(SD) maybe specified so that correction degrees for the color as thecenter color can be reflected in Equations (9) to (11). In this case, ifpoints A and B in FIG. 8 are specified, colors in 5×5 pixel ranges whosecenters are positioned at A and B are found, and a color correctioncondition setting menu for the colors is set as shown in FIG. 8. FromEquations (9)˜(11), the corrected data L4, C4, and H4 are then found.

Δl, Δc, and Δh represent how much the lightness, the chroma, and the hueof the skin color are changed according to the nonlinear tone conversionset in the fourth quadrant by the tone setting means 23, and are foundin the following manner. Based on the color data R1, G1, and B1 beforethe tone conversion and based on the color data R2, G2, B2 after thetone conversion, changes ΔL*, ΔC*, and ΔHA of the lightness L*, thechroma C*, and the hue angle HA are found for each pixel by carrying outthe processing according Equations (4)˜(8) and the processing by theinverse logarithmic conversion means 25. The changes ΔL*, ΔC*, and ΔHAare multiplied by the intensity function Wj shown in FIG. 6 as inEquations (13)˜(15) below, and Δl, Δc, and Δh are found:Δl=ΔL*×Wj  (13)Δc=ΔC*×Wj  (14)Δh=ΔHA×Wj  (15)

The sRGB conversion means 28 finds a* and b* after the correction bysolving Equations (7) and (8) by using the corrected data L4, C4 and H4.The sRGB conversion means 28 then finds tristimulus values X5, Y5, andZ5 after the correction by solving Equation (6) by using a* and b* afterthe correction and by using L*. The tristimulus values X5, Y5, and Z5are then converted into color data R4′, G4′ and B4′ according toEquation (16) below:R4′X5G4′=|A| ⁻¹ Y5B4′Z5  (16)The color data R4, G4, and B4 are then found by using the followingEquation (17), and the data are used as the color corrected image dataS4 in the sRGB color space for monitor display:R4=255×(1.055R4′^(1.0/2.4)−0.055)G4=255×(1.055G4′^(1.0/2.4)−0.055)B4=255×(1.055B4′^(1.0/2.4)−0.055)(if 0.00304≦R4′, G4′, B4′≦1)R4=255×12.92R4′G4=255×12.92G4′(if 0≦R4′, G4′, B4′<0.00304)B4=255×12.92B4′  (17)

The processing method determining means 33 calculates the number of thelattice points of the 3DLUT used for the tone conversion and the colorcorrection of the image data S0, by using the number of bits of theimage data S0. The calculated number of the lattice points of the 3DLUTis compared with the number Y0 of the pixels of the image represented bythe image data S0. If the number of the lattice points is equal to orlarger than the number of the pixels, the 3DLUT is not generated and theimage processing conditions comprising the tone conversion tables T0 andthe color correction conditions (ΔL, ΔC, and ΔH in Equations (9) to(11)) are output to the processing means 10 (the procedure shown by adashed line). On the other hand, if the number of the lattice points issmaller than the number Y0 of the pixels, the procedure goes to theprocessing to generate the 3DLUT (the processing by the printingconversion means 29 and the LUT generating means 30).

The printing conversion means 29 obtains the print image data S5 byconverting the color corrected image data S4 according to a 3DLUT forconverting the color corrected image data S4 in the sRGB color spaceinto a color space for printing.

The LUT generating means 30 finds a relationship between the color dataR0, G0, and B0 comprising the image data S0 and the color data R5, G5,and B5 comprising the print image data S5 for each of the RGB colors,and uses the relationship as the three-dimensional look-up table (3DLUT)having 33³ data to be output to the processing means 10.

The index image data S11 are input to the image processing conditiondetermining means 6, and the tone conversion processing is carried outthereon. For the index image data S11, the number of bits is not reducedand only the tone conversion processing using the tone conversion tablesT0 is carried out by the tone conversion means 22. The color correctionprocessing by the color correction means 27 is not carried out thereon,and the index image data are converted into the sRGB color space to beoutput as the index image data S11′ after the tone conversionprocessing. At this time, since the index image data S11 are not usedfor generating the 3DLUT, the tone conversion means 22 sequentiallycarries out the tone conversion on the index image data S11 according tothe tone conversion tables T0 sequentially set by the tone setting means23 while reflecting the hit of the DCMY keys 9 or the density changescaused by the correction in the tone curves, and outputs the index imagedata S11′. In this manner, the index images whose tones have beencorrected can be displayed in real time on the monitor 7.

The color correction processing may also be carried out on the indeximage data S11 in addition to the tone conversion processing. In thismanner, the index images whose tones have been converted and whosecolors have been corrected can be displayed on the monitor 7 in realtime.

The 3DLUT or the image processing conditions generated or determined bythe image processing condition determining means 6 in FIG. 1 are inputto the processing means 10. If the 3DLUT is input to the processingmeans 10, the image data S0 are converted according to the 3DLUT, andthe converted image data S12 are obtained. At this time, since the 3DLUTcomprises the 33³ data, color data comprising the converted image dataS12 are found by volume interpolation or area interpolation of the3DLUT, as has been described in Japanese Unexamined Patent PublicationNo. 2(1990)-87192, for example.

Meanwhile, if the image processing conditions comprising the toneconversion tables T0 and the color correction conditions are input tothe processing means 10, the tone conversion and the color correctionare carried out on each of the pixels in the image data S0 by using theimage processing conditions, and the image data S0 are converted intothe converted image data S12 for printing.

The number of pixels of the digital camera that obtained the image dataS0 varies, and may be smaller or larger than the number of pixelsnecessary for printing. Therefore, if the number of the pixels in theimage data S0 is larger than the pixels necessary for printing, thereduction means 11 reduces the image data S0 before the processing bythe processing means 10 and obtains the reduced image data S0′. Thereduced image data S0′ are converted into the converted image data S12.On the other hand, if the number of pixels is smaller than needed, theconverted image data S12 obtained by the processing means 10 areenlarged by the enlargement means 12, and the enlarged image data S12′are obtained.

The sharpness processing means 13 carries out sharpness processing onthe converted image data S12, or on the enlarged image data S12′according to Equation (18) below, and obtains the processed image dataS13. In Equation (18), the sharpness processing is carried out on theconverted image data S12.S13=S12+β(S12−S12us)  (18)where S12us is unsharp image data of the converted image data S12, and βis an enhancement factor.

The enhancement factor β may be changed according to a reduction ratioused by the reduction means 11 or an enlargement ratio used by theenlargement means 12.

Operation of this embodiment will be explained next. FIG. 9 is a flowchart showing the operation of this embodiment. The image data S0obtained by photographing with the digital camera are read from thememory card 2 by the reading means 3 (Step S1) The index imagegenerating means 4 generates the index image data S11 representing theindex images of the image data S0 (Step S2), and inputs the index imagedata S11 to the image processing condition determining means 6.Meanwhile, the setting information generating means 5 generates thesetting information H0 (Step S3), and inputs the setting information H0to the image processing condition determining means 6.

The tone setting means 23 of the image processing condition determiningmeans 6 sets the tone conversion tables T0 for converting the image dataS0 according to the setting information H0 (Step S4), and the toneconversion means 22 carries out the tone conversion on the index imagedata S11 based on the tone conversion tables T0 (Step S5). The indeximages without color correction are then displayed on the monitor 7(Step S6). An operator views the index images, and, if necessary (StepS7: YES), carries out the input by using the input means 8 or the DCMYkeys 9 (Step S7). The tones and/or the densities of the index images arethen corrected (Step S8). The procedure returns to Step S4 and theprocessing from Step S4 to Step S6, in which the tone conversion tablesT0 are newly set for the corrected tones and/or the densities and theindex image data S11 are subjected to the tone conversion according tothe newly set tone conversion tables T0 to be displayed on the monitor7, is repeated.

If no correction is carried out, or when the correction is completed, aresult at Step S7 becomes negative. The number of the lattice points ofthe 3DLUT calculated based on the number of bits of the image data S0 iscompared with the number Y0 of the pixels in the image represented bythe image data S0 (Step S9). If the number of the lattice points of the3DLUT is equal to or larger than the number Y0 of the pixels (Step S9:YES), the image processing conditions (the tone conversion tables T0 andthe color correction conditions) are input to the processing means 10(Step S15). If the number of the lattice points is smaller than thenumber Y0 of the pixels (Step S9: NO), the tone conversion is carriedout on the image data S0 according to the tone conversion tables T0 setfinally (Step S10), and the color correction is then carried out (StepS11). The conversion to the sRGB color space and the conversion to thecolor space for printing are also carried out (Step S12) and the printimage data S5 are obtained. The LUT generating means 30 generates the3DLUT (Step S13) by finding the relationship between the image data S0and the print image data S5 for each of the RGB colors. The 3DLUT isoutput to the processing means 10 (Step S14).

As has been described above, the processing means 10 obtains theconverted image data S12 by converting the image data S0 according tothe 3DLUT, if the 3DLUT is input thereto. However, if the imageprocessing conditions comprising the tone conversion tables T0 and thecolor correction conditions are input, the processing means 10 obtainsthe converted image data S12 by carrying out the tone conversion and thecolor correction on each of the pixels of the image data S0 according tothe image processing conditions.

The converted image data S12 are then subjected to the sharpnessprocessing by the sharpness processing means 13, and output as the printP from the printer 14.

The image data S0 read from the memory card 2 are subjected to thereduction processing by the reduction means 11 if necessary, and thereduced image data S0′ are obtained. In this case, the reduced imagedata S0′ are used as the image data S0 described above, and the sameprocessing as has been described above is carried out thereon.Meanwhile, if enlargement is necessary, the converted image data S12obtained from the image data S0 are enlarged by the enlargement means12, and the enlarged image data S12′ are obtained. The same processingis then carried out on the enlarged image data S12′ as has beendescribed above.

As has been described above, in this embodiment, the 3DLUT used forcarrying out the tone conversion processing and the color correctionprocessing on the image data S0 obtained by the digital camera isgenerated, and the processed image data S13 are obtained by convertingthe image data S0 according to the 3DLUT. Therefore, appropriate toneconversion processing and color correction processing can be carriedout, and the processed image data S13 enabling reproduction of the imagein a higher quality can be obtained.

Since the 3DLUT is generated for each of the models of digital camera,the processed image data S13 enabling reproduction of the high qualityimage not affected by the tone processing carried out in the digitalcamera can be obtained, regardless of the model of the digital camera.

Furthermore, by determining the number of the lattice points inaccordance with the number of bits of the image data S0, operation timefor generating the 3DLUT and accuracy of operation for interpolating thelattice points at the time of converting the image data S0 can becontrolled. In this manner, fast or accurate conversion of the imagedata S0 can be carried out.

Moreover, the 3DLUT is generated in the case where the number Y0 of thepixels of the image represented by the image data S0 is larger than thenumber of the lattice points in the 3DLUT, and the tone conversionprocessing and the color correction processing is carried out accordingto the 3DLUT. In the case where the number Y0 of the pixels is equal toor smaller than the number of the lattice points, the tone conversionprocessing and the color correction processing is carried out on each ofthe pixels. In this manner, the tone conversion processing and the colorcorrection processing can be carried out on the image data S0 with asmaller amount of calculation.

In the above embodiment, the image output apparatus reads the image dataS0 from the memory card 2. However, the present invention is applicableto a system in which image data are received from a transmittingapparatus via a network such as a LAN or WAN.

1. An image processing method for obtaining processed image data bycarrying out tone conversion processing and color correction processingon image data obtained by a digital camera, the image processing methodcomprising the steps of: generating a three-dimensional look-up tablefor carrying out the tone conversion processing and the color correctionprocessing simultaneously on the image data; obtaining the processedimage data by converting the image data according to thethree-dimensional look-up table; and setting a number of lattice pointsin the three-dimensional look-up table according to a number of bits ofthe image data.
 2. An image processing method for obtaining processedimage data by carrying out tone conversion processing and colorcorrection processing on image data obtained by a digital camera, theimage processing method comprising the steps of: generating athree-dimensional look-up table for carrying out the tone conversionprocessing and the color correction processing simultaneously on theimage data; obtaining the processed image data by converting the imagedata according to the three-dimensional look-up table; comparing thetotal number of pixels in an image represented by the image data withthe number of lattice points in the three-dimensional look-up table; thestep of generating the three-dimensional look-up table being a step ofgenerating the three-dimensional look-up table in the case where thetotal number of the pixels is larger than the number of the latticepoints; and the step of obtaining the processed image data being a stepof obtaining the processed image data by converting the image dataaccording to the three-dimensional look-up table in the case where thetotal number of the pixels is larger than the number of the latticepoints, and by carrying out the tone conversion processing and the colorcorrection processing on each of the pixels in the image represented bythe image data in the case where the total number of the pixels is equalto or smaller than the number of the lattice points.
 3. An imageprocessing method for obtaining processed image data by carrying outtone conversion processing and color correction processing on imagedata, the image processing method comprising the steps of: comparing anumber of lattice points in a three-dimensional look-up table used forcarrying out the tone conversion processing and the color correctionprocessing on the image data with the total number of pixels in an imagerepresented by the image data; generating the three-dimensional look-uptable and obtaining the processed image data by converting the imagedata according to the three-dimensional look-up table in the case wherethe total number of the pixels is larger than the number of the latticepoints; and obtaining the processed image data by carrying out the toneconversion processing and the color correction processing on each of thepixels in the image represented by the image data, in the case where thetotal number of the pixels is equal to or smaller than the number of thelattice points.
 4. An image processing method as defined in claim 3,further comprising a step of setting the number of lattice points in thethree-dimensional look-up table according to the number of bits of theimage data.
 5. An image processing apparatus for obtaining processedimage data by carrying out tone conversion processing and colorcorrection processing on image data obtained by a digital camera, theimage processing apparatus comprising: three-dimensional look-up tablegenerating means for generating a three-dimensional look-up table usedfor carrying out the tone conversion processing and the color correctionprocessing simultaneously on the image data, wherein thethree-dimensional look-up table generating means sets the number oflattice points of the three-dimensional look-up table according to thenumber of bits of the image data; and processing means for obtaining theprocessed image data by converting the image data according to thethree-dimensional look-up table.
 6. An image processing apparatus forobtaining processed image data by carrying out tone conversionprocessing and color correction processing on image data obtained by adigital camera, the image processing apparatus comprising:three-dimensional look-up table generating means for generating athree-dimensional look-up table used for carrying out the toneconversion processing and the color correction processing simultaneouslyon the image data; processing means for obtaining the processed imagedata by converting the image data according to the three-dimensionallook-up table; the three-dimensional look-up table generating meansbeing means for comparing the total number of pixels of an imagerepresented by the image data with the number of lattice points in thethree-dimensional look-up table, and for generating thethree-dimensional look-up table if the total number of the pixels islarger than the number of the lattice points; and the processing meansbeing means for obtaining the processed image data by converting theimage data according to the three-dimensional look-up table if the totalnumber of the pixels is larger than the number of the lattice points,and for obtaining the processed image data by carrying out the toneconversion processing and the color correction processing on each of thepixels of the image represented by the image data if the total number ofthe pixels is equal to or smaller than the number of the lattice points.7. An image processing apparatus for obtaining processed image data bycarrying out tone conversion processing and color correction processingon image data, the image processing apparatus comprising:three-dimensional look-up table generating means for comparing a numberof lattice points in a three-dimensional look-up table used for the toneconversion processing and the color correction processing on the imagedata with the total number of pixels in an image represented by theimage data, and for generating the three-dimensional look-up table inthe case where the total number of the pixels is larger than the numberof the lattice points; and processing means for obtaining the processedimage data by converting the image data according to thethree-dimensional look-up table in the case where the total number ofthe pixels is larger than the number of the lattice points, and forobtaining the processed image data by carrying out the tone conversionprocessing and the color correction processing on each of the pixels inthe image represented by the image data, in the case where the totalnumber of the pixels is equal to or smaller than the number of thelattice points.
 8. An image processing method as defined in claim 7,wherein the three-dimensional look-up table generating means sets thenumber of the lattice points in the three-dimensional look-up tableaccording to the number of bits of the image data.
 9. Acomputer-readable recording medium storing a program to cause a computerto execute an image processing method for obtaining processed image databy carrying out tone conversion processing and color correctionprocessing on image data obtained by a digital camera, the programcomprising the procedures of: generating a three-dimensional look-uptable for carrying out the tone conversion processing and the colorcorrection processing simultaneously on the image data; obtaining theprocessed image data by converting the image data according to thethree-dimensional look-up table; and setting the number of latticepoints in the three-dimensional look-up table according to the number ofbits of the image data.
 10. A computer-readable recording medium storinga program to cause a computer to execute an image processing method forobtaining processed image data by carrying out tone conversionprocessing and color correction processing on image data obtained by adigital camera, the program comprising the procedures of: generating athree-dimensional look-up table for carrying out the tone conversionprocessing and the color correction processing simultaneously on theimage data; obtaining the processed image data by converting the imagedata according to the three-dimensional look-up table; comparing thetotal number of pixels in an image represented by the image data withthe number of lattice points in the three-dimensional look-up table; theprocedure of generating the three-dimensional look-up table being theprocedure of generating the three-dimensional look-up table in the casewhere the total number of the pixels is larger than the number of thelattice points; and the procedure of obtaining the processed image databeing the procedure of obtaining the processed image data by convertingthe image data according to the three-dimensional look-up table in thecase where the total number of the pixels is larger than the number ofthe lattice points, and by carrying out the tone conversion processingand the color correction processing on each of the pixels in the imagerepresented by the image data in the case where the total number of thepixels is equal to or smaller than the number of the lattice points. 11.A computer-readable recording medium storing a program to cause acomputer to execute an image processing method for obtaining processedimage data by carrying out tone conversion processing and colorcorrection processing on image data, the program comprising theprocedures of: comparing a number of lattice points in athree-dimensional look-up table used for carrying out the toneconversion processing and the color correction processing on the imagedata with the total number of pixels in an image represented by theimage data; generating the three-dimensional look-up table and obtainingthe processed image data by converting the image data according to thethree-dimensional look-up table in the case where the total number ofthe pixels is larger than the number of the lattice points; andobtaining the processed image data by carrying out the tone conversionprocessing and the color correction processing on each of the pixels inthe image represented by the image data, in the case where the totalnumber of the pixels is equal to or smaller than the number of thelattice points.
 12. A computer-readable recording medium as defined inclaim 11, the program further comprising a procedure of setting thenumber of lattice points in the three-dimensional look-up tableaccording to a number of bits of the image data.